1. Field of the Invention
The present invention relates to conductive metal powders having excellent resistance to oxidation and minimum electromigration. They are used for electromagnetic shielding and for preventing static charges or, alternatively, as conductive materials, etc. The present invention also relates to a process for preparation and use of the conductive metal powders.
2. Related Art
As conductive powders, silver, copper, nickel, cobalt, iron, stainless steel, carbon or silver-plated copper powders are known (Japanese Patent Publication No. 47-3019, Japanese Patent Application Laid-Open Nos. 60-243277 and 61-163975).
There are problems associated with each of the conductive powders known to the prior art. For example, silver is a rare metal and expensive and also susceptible to electromigration. Copper, nickel, cobalt, iron and stainless steel suffer from serious reduction in conductivity. The reduction in conductivity results from oxidation of the surface material. Conductivity of carbon is insufficient. Silver plating not only involves complicated and troublesome plating steps but also plated silver tends to deteriorate quickly and peel-off due to poor adhesion of silver and is susceptible to electromigration.
U.S. Pat. No. 3,305,356 discloses a method for preparing powders by atomizing a silver-copper eutectic alloy (72% by weight silver) in a nitrogen atmosphere at a temperature of 875.degree. C. at 100 p.s.i., which was the upper limit of pressure for a gas atomization apparatus at that time.
Powders of alloys of silver, tin, and copper, used for dental amalgams, are known. For example, in U.S. Pat. No. 3,871,876 (corresponding to Japanese Patent Publication No. 54-35860), there is disclosed a method for preparing spheroidal powders which comprises gas atomization of a molten mass composed of silver, tin and copper using a gaseous stream (advantageously an inert gas stream), following the teaching of U.S. Pat. No. 3,253,783. U.S. Pat. No. 3,253,783 discloses that the pressure of atomized gas is in the range of 70 to 1000 p.s.i.
According to U.S. Pat. No. 3,871,876 molten fine-sized droplets of the desired composition are cooled in a manner to effect progressive solidification during cooling, commencing on the surface and moving inwardly thereof. Tin solidifies in the inner part in a high concentration since its melting point is lower than that of each of the other metals present. As a result, the outer surface of each particle contains a higher concentration of copper and silver and thus has a higher melting point, whereas the tin-rich phase, which has a lower melting point, is concentrated in the inner part of each particle. When the particles are washed with a hydrochloric acid solution, tin present on the surface is dissolved from the outer surface so that the concentration of silver and copper on the surface increases and, at the same time, the surface of the particles becomes spongy in appearance. {Since the ionization tendency decreases in the order of tin, copper and silver [cf., e.g., RIKAGAKU JITEN (Encyclopedia of Physics and Chemistry), 4th edition, page 64, published by IWANAMI Publishing Co., Tokyo, Japan], when the particles are washed with an aqueous hydrochloric acid solution, tin is dissolved mainly from the surface in the largest amount among the three kinds of metals; copper is dissolved in a less amount than that of tin; and silver is not dissolved at all}. In the powders prepared by this method, it is said that the surface of the particle is silver-rich, the inner part is tin-rich and copper is uniformly distributed in general. Presumably, the structure of alloy powders disclosed in this prior art would be derived from the procedure stated below: As droplets of a molten metal begin to solidify at their surface and progressively toward the core, silver and copper (which solidify at an early stage, owing to their higher melting points) solidify at the surface, and tin, which solidifies at a later stage owing to its lower melting point, solidifies in the inner part. In the succeeding washing procedure with an aqueous hydrochloric acid solution, metals in the surface dissolve out in the order of Sn&gt;Cu&gt;Ag=0 in accordance with a decrease in the ionization tendency. Since both silver and copper solidify on the surface in a higher concentration during the course of the gas atomization procedure, the difference between silver and copper in their distribution in the particle is presumably caused during the course of washing with hydrochloric acid.